I have an older  (09/27/2007) Dell 5150 desktop, with a Pentium 4 processor.  Starting about a week ago, if I had several programs open and tried to open an excel spreadsheet or view a video, the computer would automatically restart on it's own.  I was told it could be overheating, so I took it apart , cleaned it very well and the problems continues.  It now happens about 6 to 10 times per day.  Can anyone help me to diagnos the problem OR shoud I contact Dell support and pay to a single support ticket?

I'd appreciate any help.  Thank you.